1. Field of the Invention
The invention relates to the improvement of an input circuit used in an apparatus for measuring an electric signal such as an FFT analyzer represented by an oscilloscope, digitizer for sampling a waveform, or the like.
2. Description of the Related Art
FIG. 1 is a circuit diagram showing an example of an input circuit of a conventional apparatus for measuring an electric signal. The input circuit comprises an input coupling switching section at the front stage and a composite buffer amplifier at the post stage.
In FIG. 1, an electric signal to be measured is given to one input terminal X of a switch S3 for ground switching through an input connector 1. Another input terminal Y of the switch S3 is connected to the ground. The output of the switch S3 is directly connected to one input terminal X of a switch S1 and is also connected to another input terminal Y of the switch S1 through an AC coupling capacitor C1. The switch S1 is a switch for switching the input coupling. When the movable member of the switch S1 is connected to the terminal X, DC coupling is performed. When the movable member is connected to the terminal Y, AC coupling is executed. The output of the switch S1 is connected to an input of a switch S2a. One output terminal X of the switch S2a is directly connected to one input terminal X of a switch S2b, while the other output terminal Y or the switch 52a is connected to the other input terminal Y of the switch S2b through an attenuator 3. The switches S2a and S2b interlockingly function as a switch for switching an attenuation ratio. An input coupling switching section (input signal path) 10 is formed by the above construction.
The output of the switch S2b is connected through a capacitor C2 to the gate of a buffer transistor TR1 serving as, for example, a source-follower FET buffer. The source of the buffer transistor TR1 is connected to a power source V+, while the drain is connected through a resistor R5 to an addition point B of a low frequency signal and a high frequency signal. A resistor R4 is connected between the gate of the buffer transistor TR1 and the addition point B. A high frequency signal path (high frequency amplifying circuit), through which a high frequency component of the electric signal to be measured that was input from the input coupling switching section passes is formed by the capacitor C2, buffer transistor TR1, and resistor R5.
The output of the switch S2b, on the other hand, is also connected through a resistor R1 to, for instance, an inverting input of an amplifier A1, such as, for example, a DC operational amplifier. A non-inverting input of the amplifier A1 is connected to the ground. The output of the amplifier A1 is connected to the base of a transistor TR2 for current conversion. A feedback capacitor C3 is connected between the input and output of the amplifier A1. A lower frequency signal path (low frequency amplifying circuit) through which a low frequency component of the electric signal to be measured that was input from the input coupling switching section, is formed by the amplifier A1, resistor R1, and transistor TR2. The collector of the transistor TR2 is connected to the addition point B, while the emitter is connected to a power source V- through a resistor R7. Therefore, the low frequency signal from the low frequency signal path and the high frequency signal from the high frequency signal path are added at the addition point B. The added signal is sent to an output terminal 2 through a buffer amplifier A2 and is also supplied to the inverting input of the amplifier A1 through an inverting buffer amplifier A3 and a resistor R2. A feedback loop of the low frequency signal path is constructed by the amplifier A3 and resistor R2. A composite buffer amplifier 20 is formed by the above construction. Such a composite buffer amplifier is shown in, for example, JP-B-59-10506. The output terminal 2 is connected to a measuring section (not shown) of the apparatus for measuring the electric signal.
For example, in the case of a well-known digital oscilloscope, digitizer for sampling a waveform, or FFT analyzer, the output terminal 2 is connected to a preamplifier and an output thereof is supplied to a trigger circuit and an A/D converter, respectively. In case of a well-known analog oscilloscope, the output terminal 2 is connected to the preamplifier and an output thereof is connected to a trigger circuit system and a vertical axis circuit system, respectively.
Hitherto, when confirming a reference ground level, as shown in FIG. 1, the movable member of the switch S3 is connected to the terminal Y, an input signal line (line from the input connector 1 to a connecting point of the capacitor C2 and resistor R1) is disconnected from the input connector 1 and is connected to the ground, and the switch S1 is connected to the terminal X side.
As mentioned above, in the case where the apparatus for measuring an electric signal is, for example, an oscilloscope, a line indicative of the ground level is generated onto a display (not shown) and the position of the line is checked, thereby confirming the reference ground level.